Jostling for the key state, the Democrats stump at Sen. Harkin's annual steak fry and call for change.

Standing on a stage in a field of green, the sextet of candidates took turns Sunday singing from the same sheet music: calling for universal healthcare, a stronger labor movement, a better education system and -- in the day's most popular line -- an end to the war in Iraq.

Naturally, they disagreed on who among them could best deliver on those promises, and how.

The setting was the 30th annual steak fry hosted by Iowa's Democratic senator, Tom Harkin, an event that briefly transformed a rural fairground outside Des Moines into the most important patch of political turf west of the Potomac.

The beef wasn't the only thing barbecued.

Sen. Barack Obama of Illinois accused the Bush administration of trampling the Constitution. Sen. Christopher J. Dodd of Connecticut said the president's policies had left the United States "less safe, less secure, more vulnerable, more isolated than before."

Sen. Hillary Rodham Clinton of New York pledged to send a bipartisan team of diplomats around the world to deliver a message that "the era of cowboy diplomacy is over." Shouting over the roar of the crowd, she didn't have to identify the particular Texan she had in mind.

The event, a fundraiser for Harkin's reelection, served as an informal sort of kickoff to the fall campaign, with Iowa's first-in-the-nation presidential caucuses now less than four months off.

It also offered a chance, as Dodd put it, for "sucking up to Harkin," though Harkin's endorsement four years ago did little to help Howard Dean, the former Vermont governor who finished a disastrous third in Iowa.

This time, Harkin has vowed to stay neutral.

Rep. Dennis J. Kucinich of Ohio and former Sen. Mike Gravel of Alaska were not invited, according to Harkin aides, because neither is seriously competing in Iowa.

Obama was the marquee speaker at last year's event, introduced by Harkin as a "rock star" and forced to fend off entreaties that he run for president. His focus, Obama said then, was on the 2006 midterm elections.

On Sunday, Obama was one of the pack, allotted the same amount of time to speak -- 15 minutes -- as everyone else, and introduced by Harkin in a far less effusive fashion.

Speaking first, as determined by lot, Obama called for a transformation in Washington, not just a change of parties in the White House, and sought to minimize Clinton's insider experience.

"A lot of years in Washington doesn't guarantee good judgment," said Obama, a freshman senator who has battled suggestions that his resume is too slim for the job of president. "It doesn't guarantee good character."

Former North Carolina Sen. John Edwards echoed the criticism of Washington and the political system -- "It's rigged, it's broken, it does not work for ordinary Americans" -- and added a populist twist, suggesting big change wouldn't come by "replacing corporate Republicans with corporate Democrats."

The candidates renewed their jostling over the war in Iraq, dividing along familiar lines.

New Mexico Gov. Bill Richardson said the only solution was an immediate withdrawal of U.S. troops with no residual forces left behind.

Sen. Joseph R. Biden Jr. of Delaware urged a more cautious pullout, built around a plan to partition Iraq into three separate entities.

For her part, the front-running Clinton stayed true to form by ignoring the rest of the field and emphasizing the same goals, including a "responsible" withdrawal from Iraq.

At one point, Clinton even expressed excitement about "being the first woman president," as if it were already so.

Besides the outdoor setting, audience participation was the biggest difference from other forums the candidates have attended.

Instead of being shushed, the crowd was encouraged to hoot and cheer for their favorite candidates.

One woman was seen using a "Holler for Hillary" megaphone to root for Edwards.

Many said they could support any of the Democrats on stage.

"I just want to vote for whoever can win," said Margie Tiedje, 51, a farmer from Newton, who says she leans toward Obama but is uncommitted.

Her husband, Rick, 61, wore an Obama sticker but said much the same thing: "I like him. But if it doesn't work out, I could be happy with a whole lot of others too."